Structure determination. The program PHASER (18) was used for molecular
replacement, with the structure of unliganded PAN as a search model (Protein
Data Bank [PDB] accession no. 3EBJ) (23). Refinement was carried out with
Refmac5 (19) and PHENIX (1). An initial positional and B-factor refinement of
the molecular replacement solution led to an Rfree (4) value below 32%. The
bound metal ions and NMPs were built under the guidance of Fo-Fc difference
maps. Two peaks near Glu119 were clearly attributable to two catalytic metal
ions when the metal used in the crystallization buffer was manganese.